The present invention relates to a method for manufacturing a semiconductive resistor using a conductive layer in a semiconductor.
Various semiconductor integrated circuits are used as components in electronic equipment. It is well known that such an integrated circuit has functional elements made of various semiconductive materials. In forming such functional elements, conductive layers are formed, for example, by impurity ion implantation or by deposition of semiconductive materials containing some impurities. In this specification, a part of the conductive layer is to be used as a resistor.
Conventionally, though silicon(Si) has been used most widely as such a semiconductive material, gallium-arsenide(GaAs) has gradually become popular, and more and more studies have recently been made on semiconductive resistors using GaAs, where the electron mobility is around 5 to 10 times faster than in silicon(Si).
For example, "TEMPERATURE CHARACTERISTICS OF GaAs DIGITAL INTEGRATED CIRCUITS USING DCFL"(Ichioka et al., Technical Report SSD85-134, The Institute of Electronics and Communication Engineers of Japan, pages 45-52, Jan. 22, 1986) discloses various elemental characteristics versus temperature in case of a digital logic circuit formed using GaAs.
This article also discloses, for instance, resistance changes of a semiconductive resistor formed by disposing an ohmic electrode on a conductive layer containing n-type impurity which is formed on a semi-insulative GaAs crystalline substrate on various temperature conditions. According to the measurement results, the resistance temperature coefficient becomes +1300 ppm/.degree.C. around room temperature (measured at between -40.degree. to +80.degree. C.), and is positive going with increasing sheet resistance. It is considered that such resistance changes with positive temperature coefficients are due to the following reasons: and donor or acceptor with a certain shallow level, which generates carriers, generally releases the carriers around room temperature, and is virtually ionized. In such a case, resistivity ".rho." can be formulated with carrier density "n", unit charge "e", and mobility ".mu." as follows: ##EQU1##
As recognized from the above formula, when the resistance temperature coefficient is positive going, only mobility temperature coefficient varies with negative values, namely, mobility ".mu." becomes smaller with rising temperature. That is, in the above formula, it is seemed that unit electron charge "e" and carrier density "n" are virtually invariants in a conductive layer with a shallow level (impurity) as a sufficient stable state, and only mobility ".mu." contributes to resistance changes proportional to resistivity ".rho.".
Such conventional conductive layers with impurity levels, which constitute conventional semiconductive resistors, have the following disadvantages:
1) When a semiconductive layer is formed of a single semiconductive material, resistance temperature coefficients are obtained only as values inherent to each material though such coefficients depend on its resistivity, according to a conventional method. However, electronic circuits are normally desired to maintain some predetermined performances within wide scope of temperature. That is, in designing circuits, it is desirable to make different types of circuits of a single semiconductive material depending on required cases, that is, whether it is not preferable for semiconductive resistors, provided as various functional elements, to change their resistance values according to temperature environment by operation, or it is preferable for such resistors to change their resistance values, with a certain rate of change (temperature coefficient) determined by each design, following the change of temperature environment. In other words, if GaAs is used, as an example of such single semiconductive material mentioned above, resistance temperature coefficients are positive and no negative coefficients and zero "0" can be obtained in conventional methods.
2) As disclosed in the above mentioned document, when GaAs is used as a semiconductive material, it is easy to form a conductive layer by conventional ion implantation with sheet resistance less than around 200 (.OMEGA./.quadrature.). However, to achieve higher value of sheet resistance than 200 (.OMEGA./.quadrature.), as shown from the formula above, it is necessary either to decrease the carrier density "n" by reducing density of impurity to be implanted, or to make the conductive layer itself thinner. When a semiconductive resistor is formed by such methods, there remain such problems as poor repeatability for resistance value due to the existence of depletion layers on the surface of the conductive layer, and high current dependency of sheet resistance.